The invention relates generally to power control circuits and methods, and more particularly to high frequency/radio frequency (HF/RF) power control and monitoring circuits and methods.
Traditionally, PIN (P-type/lntrinsic/N-type) diodes are used in HF/RF applications, such as in power switching, power modulation and control. The operation of such diodes is well known to those skilled in the art. Briefly, a PIN diode has a very low capacitance when reverse biased. But when forward biased, its equivalent HF/RF series resistance becomes proportional to the direct current (DC) flowing through the diode.
A typical application circuit using a PIN diode is shown in FIG. 1. At the operating frequency f, the inductors L exhibit a series reactance xcfx89L greater than  greater than 1/xcfx89C, the series reactance of the capacitors C. The result is that the DC and the HF/RF paths are separated or isolated from each other. Therefore, in a first order approximation, the HR/RF power will not affect the PIN diode DC bias conditions.
By varying the resistance value of the resistor R or the voltage Vb, one can adjust the DC current through the diode. At the operating frequency f, the power generated by the HF/RF generator is divided between the PIN diode resistance and the load impedance Z_Load. Although impedance mismatch effects must be also taken into account, they are neglected for the purpose of this simplified illustration. By varying the DC current flowing through the PIN diode, one can adjust the HF/RF power delivered to the load impedance. Reversing the polarity of the source Vb will xe2x80x9cturn offxe2x80x9d the PIN diode, which causes the diode to exhibit a very low capacitance, thus isolating or disconnecting the HF/RF source from the load.
There are a number of disadvantages of using semiconductor diodes in high frequency applications, including high cost, large size, weak radiation hardness, low ruggedness, complexity and unreliability.
The present invention utilizes a positive temperature coefficient (PTC) resistor to offer new solutions for high frequency applications. PTC resistors have been used to protect the electric circuits from fault conditions, such as overcurrent, overload and overtemperature conditions. Typically, a PTC resistor is placed in series with a load, and under normal operating conditions, is in a low temperature, low resistance state. However, if the current through the PTC resistor increases excessively, and/or the ambient temperature around the PTC resistor increases excessively, then the PTC resistor will be xe2x80x9ctripped,xe2x80x9d i.e., converted to a high resistance state such that the current is reduced substantially to a safe level. Generally, the PTC resistor will remain in the tripped state, even if the fault condition is removed, until it has been disconnected from the power source and allowed to cool. After the current and/or temperature return to their normal levels, the PTC resistor will switch back to the low temperature, low resistance state.
An example of a PTC resistor is one which is composed of a PTC conductive polymer. The largest steady state current which will not cause any of the PTC resistors in the batch to trip is referred to as the xe2x80x9chold currentxe2x80x9d (Ihold), and the smallest steady state current which will cause all of the devices to trip is referred to as the xe2x80x9ctrip currentxe2x80x9d (Itrip). In general, the difference between Ihold and Itrip decreases slowly as the ambient temperature increases, and the higher the ambient temperature, the lower the hold current and the trip current.
By using a PTC resistor in high frequency applications, the present invention provides a number of advantages including low cost, small size, strong radiation hardness, high ruggedness, simplicity and reliability.
According to one embodiment of the present invention, a high frequency circuit comprises a capacitor, a PTC element and a resistor. The PTC element is heated by a high frequency input signal and changes its resistance. The change in the resistance of the PTC element controls the output power of the circuit.
In another embodiment of the present invention, the circuit comprises a high frequency circuit and a control circuit. The control circuit provides a DC current to the high frequency circuit to control the resistance of the PTC element, which in turn controls the output power of the high frequency circuit. In this embodiment, two separate paths are used: one for high frequency input signals and one for DC control current.
Other objects and attainments together with a fuller understanding of the invention will become apparent and appreciated by referring to the following description and claims taken in conjunction with the accompanying drawings.